Emerging Technologies Inspired by Plants - Bioinspired Approaches for Human-Centric Technologies - page 119

and mechanical characteristics of the constitutive materials. The sensors must be
sensitive to a suitable range of forces but sufficiently robust to operate in soil. The
sensors had an area of 8 m. The sensors had

the ability to detect a force in the range of 0-20 N (0-0.32 MPa). This range is
representative of the range of mechanical impedances that are experienced by some
types of plant roots, which have been reported to be between 0.24 and 0.58 MPa
(Clark et al. 2003 ).

8 mm and a total thickness of 500 μ

4.4 Bioinspiration from Materials and Motions in Plants

Plant cell walls consist of four primary components: cellulose, hemicelluloses,
lignin, and pectin. This design and variations in the hierarchical microstructure
are responsible for a wide range of mechanical properties and movements. Cellu-
lose is the main structural fiber in plants. Cellulose molecules align to form
microfibrils (with a diameter of 3-4 nm), which are aligned and bound together
into macrofibrils (with a diameter of 10-25 nm) by a matrix of hemicelluloses and
either pectin or lignin. A plant cell wall can sustain a large internal (turgor) pressure
(up to 10 atm) and can vary its stiffness for growth and motion.
Despite the absence of animal-like muscles and contractile proteins in their
tissues, plants can perform a wide range of nonmuscular movements to efficiently
explore an environment, search for nutrients and avoid possible danger, or to spread
their genetic material, which ensures continuation and diversification of the species.
These types of movements exhibit numerous appealing characteristics: high energy
efficiency (gained during the evolution process over approximately half a billion
years), high actuation force, and a wide range of motion (a successful strategy that
increases survivability in different challenging conditions). As a result of these
considerations, and since the pioneering work of Darwin ( 1875 , 1880 ), the question
of how plants move without using muscles has attracted the interest of many
scientists. From a biological perspective, the physiology of plant movements is
important to understand plant development and plant responses to environmental
stimuli, such as light and gravity (Gilroy and Masson 2008 ; Moulia and Fournier
2009 ). Additionally, an adequate understanding of these nonmuscular movements
has potential for developments in applied sciences and engineering, in particular,
the creation of new biomimetic actuation strategies related to high energy efficiency
and low power consumption (Taya 2003 ; Burgert and Fratzl 2009 ; Martone
et al. 2010 ).
Movements in plants can be characterized according to the following categories:
their nastic (movement that is independent of the spatial direction of a stimulus) or
tropic (the response of plant is influenced by the direction of a stimulus) character
and their active (live plant cells activate and control the response by moving ions
and by changing the permeability of membranes based on potential actions) or
passive (movements that are based on dead tissue that is suitable to undergo
predetermined modifications upon changes in environmental conditions) character.